\mathbb command is used to turn on blackboard-bold for uppercase letters and lowercase 'k'. If lowercase blackboard-bold letters are not available, then they are typeset in a roman font.

EXAMPLE

```
\mathbb R
```
$ \mathbb R $
```
\mathbb ZR
```
$ \mathbb ZR $
```
\mathbb{AaBbKk}Cc
```
$ \mathbb{AaBbKk}Cc $
```
\mathbb{ABCDEFGHIJKLMNOPQRSTUVWXYZ}
```
$ \mathbb{ABCDEFGHIJKLMNOPQRSTUVWXYZ} $